Context: Sticking of colliding dust particles through van der Waals forces is
the first stage in the grain growth process in protoplanetary disks, eventually
leading to the formation of comets, asteroids and planets. A key aspect of the
collisional evolution is the coupling between dust and gas motions, which
depends on the internal structure (porosity) of aggregates. Aims: To quantify
the importance of the internal structure on the collisional evolution of
particles, and to create a new coagulation model to investigate the difference
between porous and compact coagulation in the context of a turbulent
protoplanetary disk. Methods: We have developed simple prescriptions for the
collisional evolution of porosity of grain-aggregates in grain-grain
collisions. Three regimes can then be distinguished: `hit-and-stick' at low
velocities, with an increase in porosity; compaction at intermediate
velocities, with a decrease of porosity; and fragmentation at high velocities.
(..) Results: (..) We can discern three different stages in the particle growth
process (..) We find that when compared to standard, compact models of
coagulation, porous growth delays the onset of settling, because the surface
area-to-mass ratio is higher, a consequence of the build-up of porosity during
the initial stages. As a result, particles grow orders of magnitudes larger in
mass before they rain-out to the mid-plane. Depending on the turbulent
viscosity and on the position in the nebula, aggregates can grow to (porous)
sizes of ~ 10 cm in a few thousand years. We also find that collisional
energies are higher than in the limited PCA/CCA fractal models, thereby
allowing aggregates to restructure. It is concluded that the microphysics of
collisions plays a key role in the growth process.Comment: 21 pages, 15 figures. Accepted for publication in A&A. Abstract
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